LENSING GALAXIES - LIGHT OR DARK

In a recent paper, Hawkins (1997) argues on the basis of statistical s tudies of double-image gravitational lenses and lens candidates that a large population of dark lenses exists and that these outnumber galax ies with more normal mass-to-light ratios by a factor of 3:1. If corre ct, this is a very important result for many areas of astronomy includ ing galaxy formation and cosmology. In this paper we discuss our new r adio-selected gravitational lens sample, JVAS/CLASS, in order to test and constrain this proposition. We have obtained ground-based and HST images of all multiple-image lens systems in our sample and in 12 case s out of 12 we find the lensing galaxies in the optical and/or near in frared. Our success in finding lensing galaxies creates problems for t he dark lens hypothesis. If it is to survive, ad hoc modifications see m to be necessary: only very massive galaxies (M greater than or simil ar to 9 x 10(11) M.) can be dark, and the cutoff in mass must be sharp . Our finding of lens galaxies in all the JVAS/CLASS systems is comple mentary evidence which supports the conclusion of Kochanek et al. (199 7) that many of the wide-separation optically-selected pairs are physi cally distinct quasars rather than gravitational lens systems.